Metabolic acidosis is a common clinical entity that can arise from a variety of disease states, medications, and toxic ingestions. This review covers the pathophysiology, diagnosis, and management of common presentations of metabolic acidosis. We have differentiated various causes of metabolic acidosis based on the presence of a normal or elevated anion gap (AG), the sum of serum anions unaccounted for by the measurement of plasma sodium, bicarbonate, and chloride levels. Normal AG metabolic acidosis, or non-AG metabolic acidosis, arises when there is excessive loss of bicarbonate from the gastrointestinal tract or in the urine. This review covers the development and diagnosis of non-AG metabolic acidosis, including a discussion of the spectrum of renal tubular acidosis subtypes. The treatment of non-AG metabolic acidosis is reviewed. Metabolic acidosis with an elevated AG, also called AG metabolic acidosis, develops when exogenous or endogenous nonchloride acid accumulates in the body. The most common causes of AG metabolic acidosis are lactic acidosis and ketoacidosis from starvation, heavy alcohol intake, or diabetes with total body insulin depletion. Medications, toxic substances, and uremia can also lead to AG acidosis. The mechanisms and management of these causes of metabolic acidosis with high AG are covered in detail. Key words: anion-gap acidosis, diabetic ketoacidosis, lactic acidosis, non–anion gap acidosis